.TH std::pmr::polymorphic_allocator::construct 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::pmr::polymorphic_allocator::construct \- std::pmr::polymorphic_allocator::construct

.SH Synopsis
   template< class U, class... Args >                                 \fB(1)\fP \fI(since C++17)\fP
   void construct( U* p, Args&&... args );
   template< class T1, class T2, class... Args1, class... Args2 >

   void construct( std::pair<T1, T2>* p,                                  \fI(since C++17)\fP
                   std::piecewise_construct_t,                        \fB(2)\fP \fI(until C++20)\fP
                   std::tuple<Args1...> x,

                   std::tuple<Args2...> y );
   template< class T1, class T2 >                                     \fB(3)\fP \fI(since C++17)\fP
   void construct( std::pair<T1, T2>* p );                                \fI(until C++20)\fP
   template< class T1, class T2, class U, class V >                   \fB(4)\fP \fI(since C++17)\fP
   void construct( std::pair<T1, T2>* p, U&& x, V&& y );                  \fI(until C++20)\fP
   template< class T1, class T2, class U, class V >                   \fB(5)\fP \fI(since C++17)\fP
   void construct( std::pair<T1, T2>* p, const std::pair<U, V>& xy );     \fI(until C++20)\fP
   template< class T1, class T2, class U, class V >                   \fB(6)\fP \fI(since C++17)\fP
   void construct( std::pair<T1, T2>* p, std::pair<U, V>&& xy );          \fI(until C++20)\fP
   template< class T1, class T2, class NonPair >                      \fB(7)\fP \fI(since C++17)\fP
   void construct( std::pair<T1, T2>* p, NonPair&& non_pair );            \fI(until C++20)\fP

   Constructs an object in allocated, but not initialized storage pointed to by p the
   provided constructor arguments. If the object is of type that itself uses
   allocators, or if it is std::pair, passes *this down to the constructed object.

   1) Creates an object of the given type U by means of uses-allocator construction at
   the uninitialized memory location indicated by p, using *this as the allocator.
   This overload participates in overload resolution only if U is not a specialization
   of std::pair.
   \fI(until C++20)\fP

2) First, if either T1 or T2 is allocator-aware, modifies the tuples x and y to include
this->resource(), resulting in the two new tuples xprime and yprime, according to the
following three rules:
2a) if T1 is not allocator-aware (std::uses_allocator<T1,
polymorphic_allocator>::value==false) and std::is_constructible<T1,
Args1...>::value==true, then xprime is x, unmodified.
2b) if T1 is allocator-aware (std::uses_allocator<T1,
polymorphic_allocator>::value==true), and its constructor takes an allocator tag
(std::is_constructible<T1, std::allocator_arg_t, polymorphic_allocator,
Args1...>::value==true, then xprime is std::tuple_cat(std::make_tuple(std::allocator_arg,
*this), std::move(x)).
2c) if T1 is allocator-aware (std::uses_allocator<T1,
polymorphic_allocator>::value==true), and its constructor takes the allocator as the last
argument (std::is_constructible<T1, Args1..., polymorphic_allocator>::value==true), then
xprime is std::tuple_cat(std::move(x), std::make_tuple(*this)).
2d) Otherwise, the program is ill-formed.
Same rules apply to T2 and the replacement of y with yprime.
Once xprime and yprime are constructed, constructs the pair p in allocated storage as if
by ::new((void *) p) pair<T1, T2>(std::piecewise_construct, std::move(xprime),
std::move(yprime));.
3) Equivalent to construct(p, std::piecewise_construct, std::tuple<>(), std::tuple<>()),
that is, passes the memory resource on to the pair's member types if they accept them.
4) Equivalent to                                                                          (until
                                                                                          C++20)
construct(p, std::piecewise_construct, std::forward_as_tuple(std::forward<U>(x)),
                                       std::forward_as_tuple(std::forward<V>(y)))

5) Equivalent to

construct(p, std::piecewise_construct, std::forward_as_tuple(xy.first),
                                       std::forward_as_tuple(xy.second))

6) Equivalent to

construct(p, std::piecewise_construct, std::forward_as_tuple(std::forward<U>(xy.first)),
                                       std::forward_as_tuple(std::forward<V>(xy.second)))

7) This overload participates in overload resolution only if given the exposition-only
function template

template< class A, class B >
void /*deduce-as-pair*/( const std::pair<A, B>& );

, /*deduce-as-pair*/(non_pair) is ill-formed when considered as an unevaluated operand.
Equivalent to

construct<T1, T2, T1, T2>(p, std::forward<NonPair>(non_pair));

.SH Parameters

   p        - pointer to allocated, but not initialized storage
   args...  - the constructor arguments to pass to the constructor of T
   x        - the constructor arguments to pass to the constructor of T1
   y        - the constructor arguments to pass to the constructor of T2
   xy       - the pair whose two members are the constructor arguments for T1 and T2
   non_pair - non-pair argument to convert to pair for further construction

.SH Return value

   \fI(none)\fP

.SH Notes

   This function is called (through std::allocator_traits) by any allocator-aware
   object, such as std::pmr::vector (or another std::vector that was given a
   std::pmr::polymorphic_allocator as the allocator to use).

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

      DR    Applied to         Behavior as published              Correct behavior
   LWG 2969 C++17      uses-allocator construction passed     passes *this
                       resource()
   LWG 2975 C++17      first overload is mistakenly used for  constrained to not accept
                       pair construction in some cases        pairs
   LWG 3525 C++17      no overload could handle non-pair      reconstructing overload
                       types convertible to pair              added

.SH See also

   construct     constructs an object in the allocated storage
   \fB[static]\fP      \fI(function template)\fP
   construct     constructs an object in allocated storage
   \fI(until C++20)\fP \fI(public member function of std::allocator<T>)\fP
